1. Field of the Invention
The present invention relates generally to the field of antennas. More particularly, the present invention relates to scanning antennas that operate based on diffraction of an electromagnetic signal by a modulated non-equilibrium plasma grating. Specifically, a preferred implementation of the present invention relates to a millimeter wavelength (MMW) scanning antenna that operates based on diffraction of a primary beam by a modulated current-injected non-equilibrium plasma grating.
2. Discussion of the Related Art
Historically, the phased array antenna approach (quasi-optical approach) has generally been considered to be the most promising candidate for electronically controlled scanning antennas. The key components of a phased array antenna are the tunable phase-shifters (i.e., true time delay elements). However, these components are costly and often bulky.
In the past, an optical control has been used to improve the performance of phased array antennas. In this approach, infrared or visible light is used to control the electronic devices (e.g. phase-shifters) in the phased array. However, this photonics approach requires expensive photo-electronic (photonic) elements for conversion of the control signals being routed to each of the electronic devices in the array. The large number of photonic elements required for even a modest size array makes the resulting system unaffordable for most applications. This is particularly true for high frequencies (i.e., millimeter wavelength) where the manufacture of the electronic phase shifters themselves is still a challenging problem from a device fabrication perspective.
Nevertheless, the use of fiber optics technology to control an electronically scanned antenna provides a number of advantages in antenna performance. These advantages include: low interference, remote control operation, light weight, low power consumption, and high flexibility.
More recently, an optical approach rather than quasi-optical (phased array) approach was used to design photonically controlled antennas. In the optical approach, no discrete elements, phase-shifters, photo-detectors, etc., are needed. Instead of directing a millimeter wavelength beam through a photonically controlled array of discrete electronic elements, a reconfigurable plasma-grating is used to steer the antenna beam. A photo-induced plasma is excited in a semiconductor medium so as to form a periodic structure that functions as the diffraction grating. This direct approach eliminates the need for conventional phase-shifters and is a promising solution to the need to provide inexpensive beam steering in the millimeter wavelength band. The direct approach holds particular promise for such price sensitive applications as automobile collision warning systems.
Thus, a wholly optical approach, rather than quasi-optical, has been developed where a semiconductor slab containing a non-equilibrium electron-hole plasma is used as a holographic medium for diffraction of millimeter waves, thereby steering the antenna beam. Plasma patterning within the semiconductor slab defines the diffraction grating and allows the shaping of a passing millimeter wavelength beam so as to send it in a required direction. The main advantage of this direct approach is the avoidance of any need for tunable phase-shifters or other true time delay elements, thereby providing a dramatic cost reduction.
Referring to FIGS. 1A-1B, an antenna design utilizing this direct approach has been fabricated and tested in the past. Referring to FIG. 1A, a millimeter wavelength signal 10 propagates along a semiconductor waveguide 20. Alternatively, the propagation can be through a compound dielectric waveguide containing a photosensitive layer. By patterned illumination, a photo-induced plasma grating 30 (PIPG) is excited in the semiconductor waveguide 20, near its surface. The plasma grating 30 has a grating period .LAMBDA.. As in a leaky-wave antenna loaded with a metal grating, the millimeter wavelength signal 10 propagating along the semiconductor waveguide 20 interacts with the plasma grating 30 and couples out in a specific direction (i.e., at an angle .phi.) that is a function of the grating period .LAMBDA..
Referring now to FIG. 1B, the main disadvantage of this previous design is that the plasma grating 30 also significantly attenuates the millimeter wavelength signal 10 and prevents the millimeter wavelength signal 10 from propagating effectively along the entire length of the semiconductor waveguide 20. The amplitude of the transmitted millimeter wavelength signal (represented in FIG. 1B by the three parallel arrows of diminishing length) decreases as a function of the length of waveguide 20 through which the millimeter wavelength signal 10 has passed before being diffracted by grating 30. Therefore, it is very difficult to produce a radiating aperture of reasonable size with this design.